This invention relates to a method and apparatus which converts a thermometer code in an analog-to-digital converter into a binary code by using the parities of the sums of thermometer code bits in selected sets of bit positions as the bits of the binary code.
Analog-to-digital conversion is a basic procedure in communications and in every other field in which a physical quantity must be expressed in a binary code that can be processed by computers or microprocessors. The typical architectures for realizing high speed analog-to-digital converters (ADCs) are flash ADCs, interpolating and folding ADCs, two-step ADCs and pipelined ADCs. In these ADCs, a thermometer encoder is used to convert thermometer code to binary code.
Typically, a physical quantity such as the voltage value of an analog signal at any given moment of sampling is applied in parallel to a group of comparators that compare the signal to a set of equally spaced reference voltage levels. Typically, comparators associated with a reference level higher than the signal voltage put out a logic 0, while the comparators associated with a reference level lower than the signal voltage put out a logic 1. The comparator set thus translates the instantaneous signal voltage into a coded sample of the form 0 . . . 000111 . . . 1. This type of code is known as a thermometer code. In order to be usable by digital equipment, this thermometer code must be translated into a binary code in which the bits represent successive powers of 2.
Accurate real-time conversion of the thermometer code into binary code presents a number of problems. One such problem is the occurrence of bubbles, i.e. a false 0 or 1 in the thermometer code, such as 0 . . . 001011 . . . 1. Typically, a bubble is very small and affects only one thermometer digit very close to the boundary between the 0s and the 1s. Nevertheless, the position of the bubble is critical in conventional conversion schemes, so that under certain circumstances, a small bubble can cause a disproportionately large error. Another problem is time. With the demand for ever higher sampling rates, conversion schemes which involve computational steps or intermediate conversions become undesirable. Finally, circuit simplicity and power requirements are crucial in many specialty applications.
Conventional conversion schemes are illustrated in U.S. Pat. No. 4,733,220 to Knierim, in which an intermediate gray code is used to minimize the effect of the bubble errors; U.S. Pat. No. 5,382,955 to Knierim, which divides the thermometer code into subsets for encoding, and then combines the output of the subencoders to create the binary code; and U.S. Pat. No. 5,644,312 to Kenneth, in which a ROM encoder converts thermometer code to gray code, and then converts the gray code to binary code.
The present invention provides a simple, fast, error-robust thermometer-to-binary code conversion method and apparatus that occupies a very small chip area in integrated circuit devices. The inventive apparatus consumes no static power and very little dynamic power.
The invention accomplishes this by using the parities of the sums of thermometer code bits in selected sets of bit positions in thermometer code to directly form the bits of the binary code without any intermediate conversion.